The Creation and Maintenance of Wetland Ecosystems from Irrigation Canal and Reservoir Seepage in a Semi-Arid Landscape

摘要

Irrigation has increased agricultural productivity in the arid American West, and has greatly altered the natural landscape. This study sought to identify the hydro-logic processes linking irrigation canals and reservoirs to wetlands. We mapped wetlands within an irrigation company service area in northern Colorado, measured groundwater levels, and used stable oxygen isotopes to identify ground-water sources. We related vegetation composition in study wetlands to environmental variables to identify the types of wetlands supported by leakage from irrigation conveyance and storage structures. Ninety-two percent of wetlands were visually connected to the irrigation infrastructure. Wetland water tables varied with adjacent canal flow, and isotopic data indicated that wetlands within the study area were recharged solely by canal leakage. Wetland vegetation composition was related to both soil salinity and groundwater depth. Salt flats formed in areas with high salinity, marsh communities in areas with low salinity and higher standing water, and meadow communities in areas with low salinity and water tables near the ground surface. Though land conversion and water diversions have led to dramatic reductions in historic wetland area in some places, it is clear from our study that current agricultural landscapes create wetlands that rely on irrigation water.